The present invention relates in general to switching regulators and, more particularly, to a switching regulator circuit with a quick-start feature and overvoltage protection.
Switching power supplies find uses in a myriad of applications, including lighting ballast and power factor correction, for converting an AC or DC input voltage to a DC output signal. A switching power supply may utilize a pulse width modulator to rapidly switch a power transistor in the principal conduction path. The pulse width driving the power transistor determines the DC output voltage. A switching regulator control IC monitors the output voltage with an error amplifier and corrects the pulse width modulation to maintain the desired average output voltage to the load.
Conventional switching regulators that utilize a pulse width modulator with a "soft start" option must undergo a start-up delay when the circuit is powered up to establish a minimum loop regulation voltage for the error amplifier to begin controlling the pulse width modulator. After the start-up delay, the regulator increases the loop regulation voltage during the "soft start" phase as it approaches steady state operation. Prior art applications typically charge an external capacitor at a relatively slow rate to develop the minimum loop regulation voltage. The start-up delay can be quite long depending on the charging current source and the value of the external capacitor. The charging current source to the external capacitor operates continuously and must be keep small to minimize power consumption. Thus, there is a trade-off between start-up delay and power consumption.
Another problem with the prior art switching regulator circuit is the need to detect an output overvoltage condition during initial power-up or when the load is removed. The output voltage tends to shoot up if the load is removed. Therefore, it is desirable to sense the overvoltage condition and disable the switching power transistor as quickly as possible. The error amplifier and normal regulation loop operate at a low frequency for stability and therefore cannot respond in time to an overvoltage condition to shut down the switching regulator before possible damage occurs.
Hence, a need exists for an improved switching regulator for power supply control, or power factor control using low bandwidth (20 Hz), that eliminates start-up delay during power-up and further capable of quickly shutting down the switching power transistor should an overvoltage condition occur at the output.